An impact energy absorbing member that buckles by an impact load in a collision to absorb collision energy is attached to a vehicle. As one such impact energy-absorbing member, a crash box is known. A crash box includes a tubular body that is disposed in the front and rear of a vehicle, being oriented in a front-back direction. The crash box includes a set plate (an attachment plate) that is welded to this tubular body at one end portion in the longitudinal direction of the tubular body by butt arc welding or the like.
A crash box is required to repeatedly and stably buckle in its axis direction when loaded with a collision load in the axis direction of the tubular body of the crash box. The present applicants have disclosed in, for example, Patent Document 1, a patented invention relating to a crash box that includes a tubular body with groove portions. The groove portions are each provided in such a manner as to project inward from a long side of a polygon that forms the cross section of the crash box, and to extend in the axis direction of the crash box. This crash box is ensured to, by an impact load loaded in the axis direction, repeatedly buckle and to be plastically deformed into a bellows so as to absorb impact energy.
In recent years, in order to reduce the weight of a vehicle, there has been considered the reduction in the sheet thickness of a tubular body constituting a crash box. However, if the sheet thickness of the tubular body of the crash box disclosed in Patent Document 1 is reduced to about 1.2 to 1.4 mm or smaller, the end portion of the tubular body is burned through by heat input in butt arc welding with a set plate, raising the risk of declining the strength of a weld zone.
The present applicant has proposed, in Patent Document 2, a crash box that has one end portion in the axis direction of a metallic tubular body, the one end portion being folded back 180 degrees to be formed into a folded portion.